The present invention relates to a pixel circuit that performs current driving of a load element disposed in each pixel. The present invention also relates to a display device having such pixel circuits arranged in the form of a matrix, and particularly to a so-called active matrix type display device that controls an amount of current passed through a load element such as an organic EL light emitting element or the like by an insulated gate type electric field effect transistor provided within each pixel circuit.
An image display device, for example, a liquid crystal display, has a large number of liquid crystal pixels arranged in the form of a matrix and displays an image by controlling the intensity of transmitted or reflected incident light in each pixel according to image information to be displayed. While this is true for an organic EL display using an organic EL element in a pixel or the like, the organic EL element is a self light emission element unlike a liquid crystal pixel. Thus, the organic EL display has advantages of, for example, higher image visibility, no need for a backlight, and higher response speed as compared with a liquid crystal display. The brightness level (gradation) of each light emitting element can be controlled by the value of a current flowing through the light emitting element. The organic EL display differs greatly from the liquid crystal display and the like in that the organic EL display is of a so-called current control type.
As with the liquid crystal display, there is a simple matrix system and an active matrix system as the driving system of the organic EL display. The former system offers a simple structure but presents, for example, a problem of difficulty in the realization of a large and high-definition display. Therefore, development in the active matrix system is now being actively performed. This system controls a current flowing through a light emitting element within each pixel circuit by an active element (commonly a thin-film transistor (TFT)) provided within the pixel circuit. The active matrix system is described in the following documents.    [Patent Document 1]    Japanese Patent Laid-Open No. 2003-255856    [Patent Document 2]    Japanese Patent Laid-Open No. 2003-271095
Pixel circuits are disposed at respective parts where scanning lines, in the form of rows, and signal lines, in the form of columns, intersect each other in related art. Each pixel circuit includes at least a thin-film type sampling transistor, a retaining capacitance, a thin-film type drive transistor, and a load element such as a light emitting element or the like. The sampling transistor conducts between the source and the drain of the sampling transistor when the gate of the sampling transistor is selected by a scanning line and samples a video signal from a signal line. The sampled signal is written to the retaining capacitance and then retained by the retaining capacitance. The gate of the drive transistor is connected to the retaining capacitance, and one of the source and the drain of the drive transistor is connected to the load element such as a light emitting element or the like. The gate of the drive transistor receives a source-reference gate voltage based on the signal potential retained in the retaining capacitance. The drive transistor passes a current between the source and the drain according to the gate voltage, and thus passes the current through the light emitting element. The brightness of the light emitting element is generally proportional to the amount of current passed through the light emitting element. Further, the amount of current passed by the drive transistor is controlled by the gate voltage, that is, the signal potential written to the retaining capacitance. The light emitting element thus emits light at a brightness corresponding to the video signal.
The operation characteristic of the drive transistor is expressed by the following equation:Ids=(1/2)μ(W/L)Cox(Vgs−Vth)2 
In the transistor characteristic equation, Ids denotes a drain current. Vgs denotes a voltage applied to the gate with the source as a reference. Vth denotes a threshold voltage of the transistor. Another symbol μ denotes the mobility of a semiconductor thin film forming a channel in the transistor. W denotes a channel width. L denotes a channel length. Cox denotes a gate capacitance. As is clear from this transistor characteristic equation, when the thin-film transistor operates in a saturation region and the gate voltage Vgs becomes higher than the threshold voltage Vth, the thin-film transistor is brought into an on state, and thus the drain current Ids flows. As is clear from the above transistor characteristic equation, when the gate voltage Vgs is constant, the same amount of drain current Ids should always flow through the light emitting element. However, there is a problem in that degradation in brightness occurs with the passage of time.